Lighter than air aircraft which operate by means of lift afforded by buoyant gas, almost always use a means for controlling the effective lift from the gas. This often entails changing the temperature of the gas (hotter gas provides greater lift and vice versa) or by means of ballast. The ballast, typically in the form of a convenient material such as sand and finely divided metal are commonly used. The ballast, provides a drag or control (i.e., extra weight) for maintaining a desired low altitude with higher altitudes being obtained by the jettisoning thereof since the lighter the aircraft, the greater the effect of the lifting gas and the higher the altitude. Ballast is jettisoned when higher altitude is necessary (when a rapid rise is required or heating of the buoyant gas in not feasible or desired) and buoyant gas is released or cooled (hotter gas occupies greater volume with increased lift) to decrease the buoyancy and to lower the altitude. It may also be desirable to use the ballast to reduce or stop descent to make up for loss in lift that may occur from gas leakage or cooling from decrease in either the amount of incident sunlight or of incident infrared radiation from the surrounding. Control of the aircraft is therefore dependent in some measure on the amount of ballast carried by the aircraft. However, once jettisoned, ballast is lost and not renewable for additional use and control. As a result, effective air loft time or flight time is limited by the amount of available ballast. It is usually prudent to land with some ballast still in the craft to make a controlled and safe landing.
As used herein the term "airship" refers to any craft equipped With propelling means through the surrounding air and which derives at least some of its lift from a lifting gas. This includes blimps and all kinds of non-rigid airships as well as airships with a rigid structure, such as those referred to as Zeppelins or dirigibles which obtain all or most of their lift from a lighter than air gas. It also includes all hybrid craft deriving lift partly from a lighter than air gas and partly by aerodynamic lift generated by moving though an atmosphere. Airships can compensate for changes in lift to some degree by using aerodynamic forces generated by moving though the air. Nevertheless, it is common practice to carry ballast in these craft to land in case of engine failure. It is also often desirable to keep an airship close to equilibrium to minimize power needed to generate aerodynamic forces on the craft.